This application claims the priority of German patent document 101 21 993.8, filed 5 May 2001 (PCT International Application No.: PCT/EP02/04017), the disclosure of which is expressly incorporated by reference herein.
The invention relates to a hybrid ignition system for internal combustion engine having 14 V or 42 V on-board power system voltages with a timed and current-controlled ignition stage having two operating phases.
In ignition systems of this type, during a first phase, a self-induction voltage for the spark breakdown is generated from the energy stored in the magnetic field of the ignition transformer. In a second phase, the ignition system with timed control of the ignition stage and superimposed current limitation generates an alternating voltage for the ignition spark so that the ignition spark is fired without interruption even when there is an increased requirement for firing voltage due to gas flowing at the spark location. The hybrid ignition system does not require any intermediate power unit.
The invention is based on an ignition system such as is described, for example, in German patent document DE 197 00 179 C2 from Bosch, which operates according to the resonant converter principle. A typical design contains an intermediate power unit which steps up the on-board power system voltage of the on-board power system generator to values on the order of magnitude of 200 V on the primary side of the ignition transformer (which is embodied as a resonant converter). A semiconductor power output stage is actuated using a control device and the current on the primary side of the ignition transformer is interrupted when a predefined, variable switch-off current is reached. The current on the secondary side of the ignition transformer corresponds to the spark current and results from the transmission ratio of the ignition transformer, specifically essentially from the primary current, the coupling factor of the ignition transformer and the square root of the ratio of the inductances on the primary side and the secondary side.
Alternating current ignition systems have the advantage over capacitively or purely inductively operating ignition systems that the ignition energy from the intermediate power unit is transmitted continuously to the ignition spark. The maximum firing period of the ignition spark is determined by the maximum power of the intermediate power unit of the ignition system. By combining spark ignition and ion current measuring technologies, closed control circuits are obtained which make it possible for the entire ignition process including the spark plug and ignition spark to be continuously monitored and operated with the smallest possible spark current, and thus lowest possible degree of spark plug erosion.
Previously described alternating current ignition systems have the disadvantage of requiring a power unit, for generating an intermediate voltage of approximately 200 V, and a resonant converter as an ignition stage. The power unit and the resonant converter give rise to additional manufacturing and installation costs.
One object of the present invention therefore, is to provide a suitable ignition system that does not require an intermediate power unit or a resonant converter, and achieves the advantages of alternating current ignition systems.
German patent document DE 42 26 246 A1 discloses an ignition system for an internal combustion engine with subsequent spark ignition. Pure current control is carried out without timed control of the switch on and switch off times. In the process spark pauses and subsequent spark ignition occur.
German patent document DE 198 40 765 A1, on the other hand, discloses a method and a circuit arrangement for an ignition system of an internal combustion engine, in which resonant ignition with a preceding self induction phase is carried out without any mention of current control or timed control.
Finally, German patent document DE 24 44 242 A1 discloses an ignition system for an internal combustion engine which generates an ignition spark or a light arc of a predefined, relatively prolonged duration for each engine cylinder and has the capability of igniting again at successive times during such a period. In the process, timed control of the primary current is carried out with superimposed current control. The latter is carried out only in each case for the first time period, and is maintained without modification during the entire ignition enable time. 500 xcexcs are provided for the first and second time periods (switch on and switch off times) so that the ignition can operate advantageously. In addition, the limiting resistance of 30 kxcexa9 is provided in the spark plug circuit owing to the long switch on time.
However, in such ignition systems, a long switch on time and a limiting resistance are necessary in the spark plug circuit, with the result that the circuit design is more complex and costly.
Accordingly, another object of the present invention is to provide an ignition system of the generic type which is of relatively simple design, and in which a switch-on time is significantly reduced.
These and other objects and advantages are achieved by the ignition system according to the invention for a 14 V or 42 V on-board power system voltage, which is applied directly to the ignition stage without an intermediate power unit. After an input signal from a superordinate engine control device, the semiconductor power output stage is switched on by an ignition control device. As a result, a current is built up on the primary side of the ignition transformer. After a predefined maximum current value is reached for the first time, the primary side of the ignition transformer is switched off for a predefined time period. In this time period, a high voltage for the spark breakdown builds up, according to the principle of self induction, at the electrodes of the spark plug which is connected at the secondary side to the ignition transformer. After the spark breakdown, the primary side of the ignition transformer is timed and current-controlled until the end of the ignition process which is predefined by the superordinate engine control device.
The timed control operates with selected, predefined time intervals in which the semiconductor power stage is alternately switched on and off. The switch-on time is selected to be so short that when the efficiency of the ignition plasma decreases owing to the limited voltage supply from the product of the on-board power system voltage and transmission ratio of the ignition transformer after a short time, a relatively high self-induction voltage is provided again during the switch-off time. The switch-on time is however selected to be of such a length that an intermittent buildup of the stored energy takes place if there is little energy stored. In order to build up high voltage for the first spark breakdown, a large amount of energy is required so that energy has to be recharged again. The switch-off time is also selected to be as short as possible so that the drop in the energy stored in the ignition transformer during the switch-off time is small. Typical values are 10-200 xcexcs for the switch-on time, and 5-50 xcexcs for the switch-off time. A current limitation is superimposed on the timed control and it switches off the primary side of the ignition transformer whenever the primary current reaches the predefined maximum value.
The maximum current limitation protects the components of the ignition system, and the on-board power system against overloading. In conjunction with a high coupling factor of the ignition transformer, the maximum current limitation also advantageously limits the ignition spark current during the switch-on time.
The following advantages are achieved with the invention:
The ignition transformer has a transmission ratio xc3xc which is greater than 100 for an on-board power system voltage of 14 V, and greater than 50 for an on-board power system voltage of 42 V. The large transmission ratio of the ignition transformer permits the on-board power system voltage to be connected directly to the ignition stage. As a result, the intermediate power unit which is customary with alternating current ignition systems and with which the on-board power system voltage is stepped up to 200 V, is advantageously dispensed with.
The resonant oscillatory circuit which is otherwise necessary with alternating current ignition systems is dispensed with as a result of the timed control of the ignition spark after the spark breakdown with repeated switching on and off of the primary side, and a coupling factor of the ignition transformer greater than 0.7. The switching on and off processes bring about an alternating current in the ignition stage, and thus also at the spark plug, according to the forward converter principle and self-induction or flyback converter principle.
By repeatedly switching on the primary current, energy is repeatedly supplied from the on-board power system into the ignition transformer. The overall energy which is necessary for igniting the fuel mixture therefore does not need to be stored as an entire energy package in the ignition coil or in an intermediate power unit. It is sufficient to store small quantities of energy in the ignition transformer in order to maintain the ignition spark. This permits a compact design of the ignition transformer and makes it possible to use rod ignition transformers, such as described, for example, in German patent documents DE 198 40 765 A1, and DE 199 62 368 which are not prior art.
As the firing period of the invention is determined by the timed control of the engine control device, and not by the energy content in the ignition transformer or an intermediate power unit as in the prior art, the firing period of the invention can be made variable. The relatively small energy content of the ignition transformer also results in a short burn-out time of the ignition spark at the end of the firing period, which in turn has a positive effect on ion current measurement. A long post-firing period falsifies the results of an ion current measurement since due to the post-firing period the measurement results are superimposed on those of the actual ion current measurement.
If the ignition spark is fanned by a stream of gas in the cylinder, the ignition system according to the invention has the capability of supplying a correspondingly high burn voltage, and of restarting the ignition spark in the vicinity of the electrodes with the necessary breakdown voltage when there are very high burning voltages. Once the spark breakdown has taken place and the burning mixture is already ionized, a significantly lower breakdown voltage is sufficient for the renewed spark breakdown. However, with the invention, this breakdown voltage is reached again whenever the primary current is switched off by the timed control, so that it is possible to re-ignite repeatedly over the entire firing period if there is a strong flow against the ignition spark.
This post-ignition reserve is advantageously built up if, during the firing period in the switch-on time a portion of the primary current is used to maintain the ignition spark and a portion of the primary current is used to build up a magnetic field in the ignition transformer.
If the ignition spark is undesirably extinguished, the ignition system according to the invention optionally has the ability to post-ignite the ignition spark. For this purpose, the connection of the semiconductor power output stage to the primary winding L1 of the ignition transformer is formed with an optional reverse blocking diode D1. The effect of the diode is that when an ignition spark ends, the self-induction voltage at L1 at the connection to D1 can oscillate from positive voltages to negative voltages and back with the natural frequency of the ignition transformer. As a result, during the switch-off time, energy is stored back in the ignition transformer. The ignition transformer is provided with a post-ignition reserve. During the switch-on time, additional energy is stored in the ignition transformer. With the stored energy, a high voltage for a renewed spark breakdown is built up on the secondary side of the ignition transformer at L2 for the spark plug during the switch-off time. The process continues up to a renewed spark breakdown.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.